Liquid crystal devices (LCD's) employing electro-optical materials that vary their transmissivity in response to an applied electrical potential are commonly used in visual displays of information in applications such as computers, televisions, calculators, radios, automobile controls, and many other products. While display devices embodying these principles are well known, problems with the manufacture of such devices are also well known. One vexing problem is the difficulty in providing consistency of the gap between the two electrodes in the LCD. Since the performance of the device is a function of the consistency of the gap, it is important to accurately control this spacing for maximum uniformity. Improper spacing produces poor optical properties and promotes rapid deterioration of the liquid crystal fluid. Because of these and other problems, the manufacture of reliable, long-life liquid crystal displays remains somewhat of an art, rather than a science.
In the prior art, as represented by FIG. 1, the gap 16 is typically set in the liquid crystal display during the perimeter sealing process. An epoxy adhesive 12 is applied to one piece of glass 10. The spacers 11 are also applied to the glass 10, and then two pieces of glass are aligned for final cure of the adhesive 12. One process involves the use of a vacuum bagging technique to hold the glass plates together and apply the pressure necessary to set the gap as the epoxy is cured. The aligned LCD is placed into a vacuum bag 14, a vacuum is drawn on the bag, and then the opening in the bag is sealed by a heated bar on the end of the bag. The vacuum in the bag creates uniform pressure (typically equal to atmospheric pressure at sea level, about 760 Torr) over the LCD, as represented by the arrows in the drawing figures. The bagged assembly is then placed into an oven for curing of the epoxy. During this time, as shown in FIG. 2, the adhesive 12 softens when heated and flows out, allowing the gap 16 to collapse down to the level of the spacers 11, thereby setting the desired gap in the LCD. During this stage, the vacuum created in the bag is maintained, and the pressure from the external atmosphere forces the two glass plates together, setting the gap. During this time, the sealant 12 also cures to form a strong bond between the two glass plates 10. After final cure of the epoxy sealant, the display assembly can be removed from the bag, and the epoxy will hold the glass together at the gap determined by the spacers.
This process works well for twisted nematic (TN) type displays at gaps of around 10 microns. However, smaller gaps, necessary for super twisted nematic (STN) type displays, are not attainable using this procedure. This is due to the amount of epoxy adhesive placed onto the glass. Typically, epoxy is dispensed onto the glass at a thickness of 50-75 microns. Because of this thickness, the initial internal gap is much greater than the final 10 micron final gap. When the gap decreases during the heating phase, the volume of gas that was in the large gap is displaced to an area outside the glass plates, but yet inside the bag. When the temperature increases in the curing oven, the collapse of the glass plates moves this displaced volume to the outside of the plates, rather than in the gap, and thus the force exerted on the LCD sandwich is reduced. Scientists and those with skill in the art will remember that as a practical consequence, the vacuum in the bag is not a perfect vacuum, and some small amount of air still resides in the bag. When this air is heated, it expands, and the pressure inside the evacuated bag increases by about a factor of 30. Therefore, if the initial vacuum level inside the bag is around 20 Torr, it would result in a vacuum approaching one atmosphere (and hence no force at all on the LCD) during epoxy reflow and cure. The net result is that small gaps cannot be attained with this technique.
Clearly, an improvement in the state of the practicing art is needed that will allow one to manufacture LCD's with very small gaps, using the vacuum bagging technique.